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### Course: 8th grade foundations (Eureka Math/EngageNY)>Unit 7

Lesson 3: Topic D: Foundations

# Radius, diameter, circumference & π

Learn how the number Pi allows us to relate the radius, diameter, and circumference of a circle. Created by Sal Khan.

## Want to join the conversation?

• Are there any tips or tricks for remembering the equations for finding the diameter and the circumference?
• The saying for circumference and area that sticks with me is "Cherry pies delicious, apple pies are too." That is, C = πd and A = πr²
• Can someone give me a problem (NOT FORMULA!) I can keep calculating to keep accurately getting digits of pi? This is just out of curiosity. I know that circumference divided by diameter should equal pi, but I can never be exact when measuring it.
Also, can this problem preferably involve whole numbers?
(Sorry if I come off as picky and demanding)
• Pi actually goes on forever, so most people just use 3.14. Here are the first 100 digits of pi, according to math.com: 3.1415926535 8979323846 2643383279 5028841971 6939937510 5820974944 5923078164 0628620899 8628034825 3421170679. It also tells you the first 1000 digits if you really want to know.
• How can I prove my math ?
I want to be a pilot but I'm not good enough in math
• well all you have to do is study math alot and keep working harder. and do your best
• how do you find area if all you have is circumference?
• divide the circumference by 3.14 or pie then divide the answer by 2 then times that answer by itself then times it by pie or 3.14 and you have the area
• Why does every radius in a circle has the same length? I don't get that and its really confusing.
• same what if i have 27/9
• "I can't do it in my head" killed me💀
• pi is a infinite number right?
• Yes it is proven it goes on forever
• this stuff is confusing
• I'm saying 😭
• do you ever have to use the entire pi or just 3.14? Also what is pi?